From an historical perspective, wind-driven turbines have been used for centuries to provide motive power for mechanical equipment such as grinding mills and pumps. Also, for many years, there have been developments and proposals for utilizing wind turbines to provide the motive force for electrical generators. This latter subject has, in the past few years, received an increasing amount of attention as a result of supply and environmental problems involved with conventional fossil and nuclear fueled generating stations. However, a problem that has retarded the acceptance of wind turbines for use by individual users is that such systems tend to be relatively expensive and thus typical pay-back periods for amortizing the cost of such installations on the basis of power savings from commercial suppliers has been on the order of twenty years. One of the primary reasons that the cost of such units is high is that the turbine must be capable of operating over a wide range of ambient wind conditions, from light breezes in the range of eight to sixteen kilometers per hour to sudden gusts and gale force winds that may be in the range of eighty to one hundred sixty kilometers per hour. Also, electrical power can be generated most efficiently by generators that are designed to operate at maximum efficiency within a relatively narrow speed range. Therefore, it is necessary to have a turbine that will be operative in relatively light winds, withstand relatively high winds, and yet operate within a relatively narrow speed range. In addition, it has been found that fast-turning turbines are more aerodynamically efficient than slow-turning turbines because the ratio of lift forces to drag forces on the turbine blades increases as rotor speed increases--that is, high speed turbines extract more power for a given turbine size as compared with low speed turbines. Thus, it is desirable to use high tip speed turbines that have lift-to-drag ratios exceeding 50:1. However, such turbines tend to overspeed when driven by winds exceeding the normal design range, for example, winds over forty kilometers per hour. Uncontrolled overspeeding can result in destruction of the turbine as well as destruction of the power-generating equipment. These factors have tended to complicate the designs of prior wind turbines with attendant increase in cost.
Because more power can be extracted from the smaller high-speed turbines, the amount of material used to construct such turbines can be lessened, thereby lowering costs. Further, because the generating system is being driven at higher speeds, smaller, lower-cost generators or alternators can be used to produce a given power output. Because the size and weight of the turbine and the size and weight of the rotating parts of the electrical power-producing equipment are relatively lower, the wind power, which is primarily a function of wind speed, necessary to initiate rotation of the system is lessened. Thus, the system will begin turning and producing power at lower wind speeds and will operate more frequently, thus increasing the amount of energy derived from the unit over a given time span.
Heretofore, efforts of several different types have been made to control wing tip speed in wind-driven turbines. Mechanical friction brakes applied either directly to the turbine axle or indirectly have been proposed for decelerating or limiting the speed of the turbine wheel, but the power converted to heat in such friction brakes is merely lost to the ambient.
Some designs employ air flow spoilers mounted on or near the wing tips that cause turbulent flow conditions and increase drag to retard forward spin. Apparatus of this type includes wings having "air brakes" that become operative after the turbine reaches a predetermined speed. The spoilers attempt to dissipate or redirect the oncoming wind, yet the wind forces still attempt to drive the turbine so that this method of speed control tends to induce wing stresses within the turbine.
Control systems are used that govern the electrical or mechanical load on the turbine to reduce rotational speed. The systems must be built into the power generator or its drive train and tend to raise the cost of the generating assembly.
Systems employing means for reorienting the turbine away from the prevailing wind direction have also been proposed. However, such systems lack responsiveness, especially with large diameter turbines, because gyroscopic and inertial forces prevent the turbine from being moved quickly enough to offer effective protection.
In other attempts, the wing angulation or pitch is varied in response to centrifugal forces imposed on movable weights that are at or near the turbine hub or on axially movable blades, so that the turbine blades can release or spill excessive wind force build-up on the front face of the wheel, thereby losing potential driving force and absorbing less power.
Designs employing these expedients are shown in U.S. Pat. No. 2,832,895 and U.S. Pat. No. 2,505,969, respectively. Such designs employ relatively complicated and expensive mechanical linkages for controlling wing pitch.